A compression-ignition internal combustion engine in which a particulate filter that collects particulate matter in exhaust gas is arranged within an exhaust passage has been known. The particulate filter includes exhaust gas inflow passages and exhaust gas outflow passages that are alternately arranged, and porous partitions that separate the exhaust gas inflow passages and the exhaust gas outflow passages from each other. A downstream end of each exhaust gas inflow passage is closed by a downstream-side plug and an upstream end of each exhaust gas outflow passage is closed by an upstream-side plug. Therefore, exhaust gas first flows into the exhaust gas inflow passages, then passes through the surrounding partitions, and flows out into the adjacent exhaust gas outflow passages. As a result, particulate matter in the exhaust gas is collected on the partitions and is thus suppressed from being released into the atmosphere.
However, if the amount of particulate matter collected by the particulate filter increases, pressure loss of the particular filter increases gradually. Consequently, the engine output may be decreased. Thus, in this internal combustion engine, a PM removing process, in which the temperature of the particulate filter is increased while the particulate filter is maintained in an oxidizing atmosphere, is performed to burn the particulate matter, thereby removing it from the particulate filter.
By the way, a non-combustible component called “ash” is contained in exhaust gas. The ash is collected along with the particulate matter on the particulate filter. However, even if the PM removing process is performed, the ash is not burnt or vaporized but remains on the particulate filter. Thus, as the operating time of the engine becomes longer, the amount of the ash collected on the particulate filter increases gradually, and the pressure loss increases gradually in the particulate filter. Consequently, output of the engine may decrease even if the PM removing process is repeatedly performed.
In view of this, a particulate filter in which a through-hole is formed in each downstream-side plug to allow the ash to flow out of the particulate filter through the through-holes has been known (see Patent Literature 1). In the Patent Literature 1, as the operating time of the engine becomes longer, the through-holes are blocked by particulate matter. When the through-holes are blocked, the particulate filter can collect particulate matter in the same manner as a conventional particulate filter that does not have any through-holes. Next, after a PM removing process is performed, the particulate matter blocking the through-hole is removed and thus the through-holes are opened. As a result, the ash on the particulate filter is discharged from the particulate filter through the through-holes.